Gas-Liquid-Solid Three-Phase Boundary in Scanning Electrochemical Cell Microscopy.

The gas-liquid-solid interface plays a crucial role in various electrochemical energy conversion devices, including fuel cells and electrolyzers. Understanding the effect of gas transfer on the electrochemistry at this three-phase interface is a grand challenge. Scanning electrochemical cell microscopy (SECCM) is an emerging technique for mapping the heterogeneity in electrochemical activity; it also inherently features a three-phase boundary at the nanodroplet cell. Herein, we quantitatively analyze the role of the three-phase boundary in SECCM involving gas via finite element simulation. Oxygen reduction reaction is used as an example for reaction with a gas reactant, which shows that interfacial gas transfer can enhance the overall mass transport of reactant, allowing measuring current density of several A/cm. The hydrogen evolution reaction is used as an example for reaction with a gas product, and fast interfacial gas transfer kinetics can significantly reduce the concentration of dissolved gas near the electrode. This helps to measure electrode kinetics at a high current density without the complication of gas bubble formation. The contribution of interfacial gas transfer can be understood by directly comparing its kinetics to the mass transfer coefficient from the solution. Our findings aid the quantitative application of SECCM in studying electrochemical reactions involving gases, establishing a basis for investigating electrochemistry at the three-phase boundary.